Project Summary Why do some cells become malignant while most others do not? Can the cell-of-origin for cancer be identified based on specific features? How do we conceptualize such specific features within the framework of cancer biology? This proposal aims to provide answers for these questions. There are two prevailing, non- exclusive models to explain how cancers arise. One is that full transformation requires accumulation of multiple genetic mutations so that malignancy occurs only when all mutations have been acquired. The other is that mutations in rare stem cells cause malignancy. Recent technologic development has brought about contradicting evidence that is difficult for either of these models to explain, demanding a new conceptual framework to account for how malignancy is initiated. My previous work in tracking how somatic cells change their fate into pluripotent stem cells demonstrated that the epigenome responds to genetic insults differently when cell cycle is greatly accelerated. Based on this discovery, I propose a third model for how malignancy arises from normal cells: a transiently ultrafast cycling cell provides oncogenes with a permissive epigenetic context to induce malignancy. This hypothesis will be tested primarily using the hematopoietic system, in which the linage relationship, surface phenotype and cell cycle behaviors have all been well charted. The oncogene of choice is MLL-AF9, a fusion oncogene involved in human acute myeloid leukemia, which requires chromatin binding to initiate malignancy. We have established novel experimental approaches and obtained preliminary data that support malignant transformation is initiated from the fastest cycling hematopoietic progenitors in vitro. Because cancer is a disease in vivo, experimental strategies to vigorously define the cancer cell-of-origin in vivo are presented. New mouse models that will enable the visualization and measurement of cell cycle speed of leukemia initiating cells in vivo under homeostasis will be generated. Following our preliminary data supporting that ultrafast cycling cells display more accessible chromatin, we will formally test whether fast cell cycle promotes transformation by providing oncogenes with a more permissive chromatin context using genomic, genetic and imaging approaches. Finally, the generalizability of the central hypothesis will be tested using additional oncogenes and in solid tumor systems. Since the ultrafast cycling state could arise transiently in developmental, physiological, pathological or therapeutic conditions, this model makes a number of important predictions that can and will be experimentally tested. I anticipate to demonstrate that cell cycle acceleration fuels transformation not by generating more mutations, but by creating a more permissive chromatin context for oncogenic transcription factors to establish malignant gene expression programs. Overall, I aim to reveal a new logic for how oncogenes act and co-operate to induce malignancy, and to provide a fundamentally different perception for cancer initiation and intervention. The Director's New Innovator Award is the only support mechanism that can make achieving this goal possible.